Synthetic Molecular Bipeds.

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DOI: 10.1002/anie.201006946
Molecular Motors
Synthetic Molecular Bipeds**
Emilio M. Prez*
dynamic covalent chemistry · macrocycles ·
molecular devices · molecular motors · photochemistry
There are relatively few species of animals that are habitual
bipeds. Evolution seems to have favored other types of
terrestrial locomotion, so that only birds and a few mammals—including humans, kangaroos, and wallabies—utilize
their two rear limbs to move from one place to another. This
fact, together with an understandable unawareness of Australian fauna, led Plato to famously define humans as
“featherless bipeds”.[1]
Among motor proteins, however, bipedalism is the
preferred mode of locomotion. Kinesins (Figure 1)[2] and
dyneins utilize two “feet” to move along microtubules, as so
Figure 1. Solid-state structure of kinesin binding adenosine diphosphate (ADP).[2] The feet are shown in different colors (blue and red) for
clarity. The red foot is bound to ADP (shown in yellow). The neck
linker to the cargo-binding domain is shown in gray.
typically move towards the plus end of microtubules, while
dyneins move towards the minus end; 3) repetitive and
progressive operation: they can repeatedly perform similar
mechanical cycles without undoing the physical task performed at each step; 4) functionality: the motion of the
proteins is exploited to carry out biologically relevant tasks.
Motor proteins transport cargoes—in the case of dyneins and
kinesins—or exert a force that results in muscle contraction—
in the case of myosins.
It is no wonder that scientists have been fascinated by such
machines, and have tried to produce artificial systems that
show similar features.[4] Until very recently, the only successful artificial systems were built with building blocks directly
borrowed from nature, and several astonishing examples of
DNA-based walkers have already been reported.[5]
Leigh and co-workers have succeeded in designing and
synthesizing small-molecule track–walker systems.[6] In order
to attain the processivity displayed by naturally occurring
motor proteins while achieving a degree of control over the
attachment and detachment of the feet, the research team
used the toolbox of reactions of dynamic covalent chemistry,[7]
in a compromise between the lability and tunability of weak
noncovalent interactions and the stability of covalent bonds.
In particular, they used disulfide (sensitive to base and/or
redox chemistry) and hydrazone (sensitive to acid) exchange
to achieve the walking motion. The structures of the track–
walker systems are shown in Scheme 1. The walker unit (red)
do myosins to move along actin filaments.[3] Some key
common features of these motor proteins are: 1) processivity:
when one foot is detached from the track to allow for
movement, the other foot remains bound to the track, so that
the protein remains attached over many steps (ca. 100 in the
case of most kinesins); 2) directionality: for instance, kinesins
[*] Dr. E. M. Prez
IMDEA Nanociencia, Facultad de Ciencias, Mdulo C-IX, 3aplanta
Avda. Fco. Toms y Valiente, 7
Ciudad Universitaria de Cantoblanco, 28049, Madrid (Spain)
E-mail: emilio.perez@imdea.org
Homepage: http://www.nanociencia.imdea.org
[**] The MICINN of Spain is acknowledged for a Ramn y Cajal
Fellowship, cofinanced by the European Social Fund.
Angew. Chem. Int. Ed. 2011, 50, 3359 – 3361
Scheme 1. Chemical structure of the track–walker systems.
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Highlights
Figure 2. Operation of track–walker systems A and B. Color codes as in Scheme 1; pentagons represent disulfide–thiol foot and footholds and
circles represent hydrazide–hydrazone foot and footholds. The placeholder is not represented for clarity. Below the arrows the intermediates are
shown.
comprises thiol and hydrazide units, which are separated by
alkyl spacers. The track unit, in turn, features four footholds:
two thiols (blue) for the formation of disulfide linkages, and
two aldehydes (green) for the formation of hydrazone bonds.
In the case of system A, the two halves of the track are
connected through a rigid triazole unit.[6a,b] On the other hand,
the track in system B features a stilbene-type olefin, which,
through E/Z photoisomerizations, allows for the control of
the distance between the two central footholds of the track, as
represented by the car jack in Scheme 1 and Figure 2.[6c]
Finally, methyl 3-mercaptopropanoate (black) is utilized as
a reference to track the movement of the walker unit.
The track–walker system A can be operated through
changes in the pH value, by addition of trifluoroacetic acid
(TFA) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Under
acidic conditions, hydrazone exchange takes place (circles in
Figure 2), and the hydrazide foot of the walker explores both
aldehyde footholds, until reaching thermodynamic equilibrium. Very importantly, under these conditions, the disulfide
linkage (pentagons in Figure 2) is stable, in order to prevent
dissociation of the walker from the track. On addition of
DBU, the hydrazone foot is locked and the disulfide bond
becomes labile. Alternating the acid and base results in a net
displacement of the walker unit to the right (Figure 2), since
the walker was initially synthesized occupying the two left
external footholds.[6a]
By varying the length of the spacer in system A, it was
found that for n = 1 and 2 the walker unit cannot actually
walk, since its strides are too short to bridge the two internal
footholds. For n = 3, 4, and 7, the acid–base switch allows the
system to walk until reaching the minimum energy distribution. Notably, in the cases of n = 3 and 4, carrying out the
disulfide exchange under kinetically controlled redox stimulation provides a means to bias the direction of motion
sufficiently to transport the walker directionally. Interestingly,
the motion of the system switches to the opposite direction
when changing from n = 3 to n = 4. On the other hand, no
directionality was observed when operating the system where
n = 7.[6b]
System B includes a stilbene unit in the track, which
provides a means to increase or decrease ring strain when the
walker unit bridges the two internal footholds. Prior to the
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first disulfide exchange reaction, an E!Z photoisomerization brings the two central footholds closer, thus inducing the
walker unit to preferentially occupy the central position to
form a less strained macrocycle (see Figure 2 B).[8] With the
walker unit in place, the double bond is restored to its
E configuration, thus increasing the ring strain and favoring
displacement of the walker unit to the right when the
hydrazone links are labilized. The net result is that the walker
is approximately 1.5 times more likely to take steps to the
right than to the left.[6c]
The molecular devices reported by Leigh and co-workers
represent the first examples of small-molecule synthetic
bipeds.[9] So far, the research team has produced systems
which show three out of four of the main characteristics of
naturally occurring walking motor proteins. The most advanced walker system (B in Figure 2) utilizes four different
stimuli in order to repeatedly take directionally biased steps
along its track without detaching from it (i.e., it shows
processivity, directionality, and repetitive operation). In
principle, such systems should also be capable of walking
along polymeric tracks. Surely, synthesizing such tracks and
devising systems that can exploit their motion to transport
cargoes—to achieve functionality—are two of the major
challenges that lie ahead.
Received: November 5, 2010
Published online: March 4, 2011
[1] In fact, Plato did so only indirectly, at least in writing: “I say that
we should have begun at first by dividing land animals into biped
and quadruped; and since the winged herd, and that alone, comes
out in the same class with man, should divide bipeds into those
which have feathers and those which have not.” Platon, Statesman
(a Dialogue), (translated by B. Jowett), Polit Press, 2010, pp. 63–
64.
[2] F. Kozielski, S. Sack, A. Marx, M. Thormahlen, E. Schonbrunn, V.
Biou, A. Thompson, E. M. Mandelkow, E. Mandelkow, Cell 1997,
91, 985.
[3] M. Schliwa, Molecular Motors, Wiley-VCH, Weinheim, 2003.
[4] For a comprehensive review on synthetic molecular machines,
see: E. R. Kay, D. A. Leigh, F. Zerbetto, Angew. Chem. 2007, 119,
72; Angew. Chem. Int. Ed. 2007, 46, 72.
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Angew. Chem. Int. Ed. 2011, 50, 3359 – 3361
[5] a) T. Omabegho, R. Sha, N. C. Seeman, Science 2009, 324, 67;
b) H. Gu, J. Chao, S.-J. Xiao, N. C. Seeman, Nature 2010, 465, 202;
c) K. Lund, A. J. Manzo, N. Dabby, N. Michelotti, A. JohnsonBuck, J. Nangreave, S. Taylor, R. Pei, M. N. Stojanovic, N. G.
Walter, E. Winfree, H. Yan, Nature 2010, 465, 206.
[6] a) M. von Delius, E. M. Geertsema, D. A. Leigh, Nat. Chem.
2010, 2, 96; b) M. von Delius, E. M. Geertsema, D. A. Leigh, D.T. D. Tang, J. Am. Chem. Soc. 2010, 132, 16134; c) M. J. Barrell,
A. G. Campaa, M. von Delius, E. M. Geertsema, D. A. Leigh,
Angew. Chem. 2011, 123, 299; Angew. Chem. Int. Ed. 2011, 50,
285.
Angew. Chem. Int. Ed. 2011, 50, 3359 – 3361
[7] P. T. Corbett, J. Leclaire, L. Vial, J.-L. Wietor, K. R. West, J. K. M.
Sanders, S. Otto, Chem. Rev. 2006, 106, 3652.
[8] For a detailed study on the opening and closing of these
macrocycles, see: M. von Delius, E. M. Geertsema, D. A. Leigh,
A. M. Z. Slawin, Org. Biomol. Chem. 2010, 8, 4617.
[9] For selected examples of nonlinear synthetic molecular motors,
see: a) R. Eelkema, M. M. Pollard, J. Vicario, N. Katsonis, B. S.
Ramon, C. W. M. Bastiaansen, D. J. Broer, B. L. Feringa, Nature
2006, 440, 163; b) S. P. Fletcher, F. Dumur, M. M. Pollard, B. L.
Feringa, Science 2005, 310, 80; c) V. Balzani, M. Clemente-Leon,
A. Credi, B. Ferrer, M. Venturi, A. H. Flood, J. F. Stoddart, Proc.
Natl. Acad. Sci. USA 2006, 103, 1178.
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